1. Field of the Invention
This invention relates to a method of estimating the angle of arrival of a signal at an antenna to an accuracy that is a small fraction of the antenna aperture beamwidth.
2. Background of the Invention
In the field of antenna design, it is known that the ratio of wavelength (WL) to antenna diameter (D), (wherein "antenna diameter" can refer to dimensions other than the diameter in the case of non-circular antennas as is well known in the art and generally is a function of the antenna area) determines the resolving power for that antenna, assuming a conventional approach.
Monopulse processing is an unconventional approach that has been used for several decades to improve the standard angular resolution limit of the antenna which is WL/D (measured in radians). Monopulse effectively divides the antenna aperture into segments. Multiple linear combinations of segment outputs are formed. Nonlinear operations on these linear combinations are performed. One output normally defines the elevation angle of arrival (AOA-el) of the signal relative to boresight and another output defines the azimuth angle of arrival (AOA-az) of the signal relative to boresight.
Consider a four quadrant aperture in accordance with the prior art, and assume the aperture quadrants are labelled A, B, C and D in a counterclockwise direction commencing with the upper left (FIG. 3) with the elevation direction being from quadrant B toward quadrant A or quadrant C toward quadrant D and the azimuth direction being from quadrant B toward quadrant C or quadrant A to quadrant D. Standard monopulse processing forms: a sum beam wherein sum=Real(A+B+C+D), an azimuth difference beam wherein DeltaAz=Imaginary((A+B)-(C+D)) and an elevation difference beam wherein DeltaEl=Imaginary((A+D)-(B+C)).
Signal detection is typically performed by amplitude testing the quantity Sum*SumConjugate where "Conjugate" has its standard mathematical meaning, namely that the imaginary portion of the signal is of opposite sign to the quantity which is not the conjugate.
Azimuth angle estimation is computed as: AzEst=(DeltaAz*SumConjugate)/(Sum*SumConjugate) for amplitude monopulse, or as AzEst=arctangent (DeltaAz/Sum) for phase monopulse.
Elevation angle estimation is computed as: ElEst=(DeltaEl*SumConjugate)(Sum*SumConjugate) for amplitude monopulse, or as ElEst=arctangent (DeltaEl/Sum) for phase monopulse.
In standard monopulse processing, a division is required in order to estimate target angle of arrival (AOA) within the main beam. Usually, this requires that the signal to noise ratio (SNR) be upward of 10 dB to insure an extremely small probability of dividing by zero or a number close to zero.
The prior art systems which use the above described functions require an additional antenna in the array, an additional manifold and receiver channel or a separate array for the purpose of blanking or cancellation of sidelobes.